This invention relates to a device for load transfer, particularly transfer of petroleum fluids, between two vessels at sea, comprising at least a loading hose arranged to be drawn between an after manifold at the stern of a first vessel and a midship manifold at a second vessel.
This invention particularly relates to a device for mooring and storage of a normally floating loading hose when the loading hose is not used for transfer of petroleum fluids from a vessel at sea, preferably a floating (production,) storage and off-loading vessel FSO/FPSO.
Load transfer between to vessel units at sea is an especially demanding task, particularly due to persisting relative movement between the vessels. A known method to perform this kind of operation is to arrange the vessels in a tandem, that is, the second vessel moored from the bow party to the stern of the first vessel. Typically this may be a so-called FPSO (Floating Production, Storage and Off-loading), while the other may be a tank vessel, particularly known as a shuttle tanker. A floating loading hose may be used for the transfer, i.e. the discharging or unloading from the FPSO and the loading of the shuttle tanker, whereas such a loading hose is constructed or arranged to float at the sea surface and to be towed on board to the shuttle tanker before load transfer. Modern shuttle tankers may have a bow manifold for charging, but a major proportion of the conventional shuttle tankers usually have a charging device consisting of a midship manifold for intake of the oil load. For this reason a relatively long loading hose is needed, from the stern on the FSO/FPSO-vessel to the midship manifold on the shuttle tanker. The separation between the vessels, i.e. between the stern of the FPSO-vessel and the bow of the shuttle tanker may be about 50 to 90 meters, and the extension of the loading hose is normally between 150 and 300 meters. The internal diameter of the loading hose will normally be between 10 and 20 inches, i.e. between 25 and 50 cm, adapted to the actual pumping rate for the oil charge. Conventionally the bullnose end of the loading hose is brought to the shuttle tanker by means of a dedicated auxiliary craft, e.g. a tender.
Obviously such loading hoses floating freely in the sea in the periods between load transfer operations, may be subject to wear and by that risk damage, both to the loading hose and its connection with the vessel, whether the weather conditions are difficult or not, but particularly due to wave action. Under the present conventional storage method where the loading hose is drifting with the weather, hanging astern from the FPSO-vessel (which also may be lying on the weather, preferably single point moored, freely rotatable on a turret) the experience has shown that the loading hose is worn due to wave action, gradually leading to loss of the buoyancy and beginning to sink, or that it is torn off and lost. This of course implies a considerable expense and a time loss, leading to delays and extra work being disproportionally expensive because of extraordinary operation out of the schedule, remotely at sea and often under heavy weather, with the problems imposed by such a replacement both with respect to logistics and with respect to working conditions, both of which relates to safety. Keeping a shuttle tanker lying waiting also leads to loss of money.
By a conventional method according to the above, where a mostly conventional tank vessel shall receive a fluid load from an FPSO, a loading hose is laid out astern from the FPSO-vessel and along the hull""s side of the tanker vessel for coupling to a midship manifold or the like, which is normally situated about midship of the tanker vessel. When the shuttle tanker leaves the FPSO-vessel after the end of loading, the bullhead of the loading hose is dropped to the sea.
On the background of the above mentioned known art the invention comprises a device for arrangement of the loading hose during the times between the load transfer operations. The new and characteristic traits are primarily comprised of a hose swivel arranged for permanent swivelled connection between one of the ends of the loading hose and an after manifold, and also mutually separated hoisting appliances with movable catch hooks adapted for the loading hose, and arranged from the after manifold forward along the hull""s side, adapted to complete or partial hoisting of the loading hose from the natural floating position.
Thus one achieves a device that gives a safe mooring possibility with the loading hose pivoted in a forward direction about a hose swivel on the after manifold and laid towards the hull""s side of the FPSO-vessel or the FSO-vessel. When the loading hose is laid parallel with the hull""s side, it may be engaged by the catch hooks of the hoisting appliances along the hull""s side. The loading hose may then be hoisted completely or partially from the sea and thus become less exposed to waves and current with the inherent wear and damage of which the loading hose otherwise would have been subject to if it were freely floating in the sea.
Among the achieved advantages by such a solution we here mention a substantially prolonged lifetime for the loading hose, less maintenance work, improved reliability for commencing the load transfer on schedule, reduced delay for shuttle tankers due to a defect loading hose, improved safety for vessels operating astern of the FPSO-vessel because no loading hose is drifting in the sea during intervals between load transfer operation times. If improved reliability for load transfer from the FPSO-vessel to the shuttle tankers is achieved, one may be able to keep a more even and higher production of oil from the reservoir.
The favourable mooring conditions achieved for the loading hose obviously depend on the number of hoisting appliances, their location on the deck and their mutual separation, the design of the catch hook devices and how high out of the sea one may, or wish, to hoist the loading hose. Clearly, under fair weather conditions it may suffice only to engage the catching devices to the loading hose, and hoisting it only partially up from the sea. On the contrary, it may, during heavy weather or -wave conditions, be necessary to hoist the loading hose completely up and above the sea in order to avoid damage on, or loss of, the loading hose.
As will emerge from the following, it may for certain conditions and operations, be safeguarded considerable additional advantages by using a winch or hoisting crane arranged leading in the direction of the bow with respect to the leading crane, and arranged to winch and tighten up the loading hose by means of a hauling line attached near the free (bullnose) end of the loading hose.
The solutions comprised by the invention may be adapted for several modes of operation, such as:
An essentially stationary vessel of the FSO or FPSO type in harsh weather areas, possibly also in less demanding weather areas.
The load to be transferred is oil.
Transfer of the oil and return of gas (VOCxe2x80x94Volatile Organic Compounds).
As for this, it must be mentioned that particularly during the waiting interval between the previously described known tandem operations of load transfer between two vessels under weather conditions or situations as mentioned in the previous chapter, the known holding position and imperfect mooring methods often implies considerable risks as for wear, sinking and average of the loading hose.
Such unwanted incidences will, by means of the device according to the invention, be substantially eliminated by ensuring that the loading hose is hoisted or lifted mostly out of the sea during periods when it is not used for load transfer. The pivoting turning to the forward pointing direction can be performed by means of lines or wires drawn between the FPSO-vessel and the loading hose, but in a preferred embodiment of the invention be performed by means of an auxiliary craft or tender. It is also possible to bring the loading hose towards the FPSO hull""s side by rotating it by means of the FPSO-vessel""s own engine force.